1. Technical Field of the Invention
This invention relates generally to communication systems and more particularly to RFID communications.
2. Description of Related Art
A radio frequency identification (RFID) system generally includes a reader, also known as an interrogator, and a remote tag, also known as a transponder. Each tag stores identification data for use in identifying a person, article, parcel or other object. RFID systems may use active tags that include an internal power source, such as a battery, and/or passive tags that do not contain an internal power source, but generate power from radio frequency (RF) signals received from a reader.
In general, to access the identification data stored on an RFID tag, the RFID reader generates a modulated RF interrogation signal designed to evoke a modulated RF response from the tag. The RF response from the tag includes the coded identification data stored on the RFID tag. The RFID reader decodes the coded identification data to identify the person, article, parcel or other object associated with the RFID tag. For passive tags, the RFID reader may also generate an unmodulated, continuous wave (CW) signal from which the passive tag derives its power.
RFID systems typically employ either far-field technology, in which the distance between the reader and the tag is great compared to the wavelength of the carrier signal, or near-field technology, in which the operating distance is less than one wavelength of the carrier signal. In far-field applications, the RFID reader generates and transmits an RF signal via an antenna to all tags within range of the antenna. One or more of the tags that receive the RF signal responds to the reader using a backscattering technique in which the tags modulate and reflect the received RF signal. In near-field applications, the RFID reader and tag communicate via mutual inductance between corresponding reader and tag inductors.
In RFID systems that include passive tags, a passive tag's ability to generate power from a received RF signal and/or a mutual inductance signal (hereinafter collectively referred to as an RFID signal) directly correlates to the power level at which the tag receives the signal. The power level of the RFID signal is maximized when the reader and tag have an ideal orientation. For example, for near-field applications, an ideal orientation occurs when the inductor of the reader is parallel to the inductor of the tag. In many near-field applications, however, the reader is a handheld device that is swiped proximal to the tag. In such instances, the ideal orientation is rarely achieved and, as the orientation approaches perpendicular, less and less energy is transferred from the reader's inductor to the tag's inductor.
Therefore, a need exists for an RFID interface that provides improved energy transfer between the reader and the tag.